Wavelength multiplex transmission systems

ABSTRACT

A wavelength multiplex transmission system is provided to reduce crosstalk among wavelengths and reduce deterioration of signals. At a transmission apparatus, one input signal is differentially divided into two; each of the two is converted to an optical signal; and then they are wavelength-multiplexed and transmitted. When crosstalk is arisen in the wavelength multiplex transmission system, the crosstalk is superposed on the two optical signals. The crosstalk is equally superposed on each of the signals with inverted polarities. Accordingly, converting the optical signals to electrical signals and then differentially combining them at a receiving apparatus, their signal components are accumulated, while their crosstalk components are cancelled out. Thereby, in the wavelength multiplex transmission system, deterioration of optical signals due to crosstalk can be reduced.

TECHNICAL FIELD

The present invention relates to a wavelength multiplex transmissionsystem used for transmission of a wavelength-multiplexed optical signal,and a transmission apparatus and a receiving apparatus to be applied tosuch wavelength multiplex transmission system.

BACKGROUND ART

FIG. 1 shows an example of configuration of a typical wavelengthmultiplex transmission system (e.g. see non-patent literatures 1 to 8).This is an example where three wavelengths are wavelength-multiplexed.Three electrical signals 7 a, 7 b and 7 c inputted from input terminals191, 192 and 193, are converted to optical signals 7A, 7B and 7C withdifferent wavelengths by optical transmitters 111, 112 and 113,respectively. These optical signals are wavelength-multiplexed bywavelength multiplex filter 131 and converted to onewavelength-multiplexed signal, which is transmitted via opticaltransmission line 151. The transmitted wavelength-multiplexed signal isseparated by wavelength separation filter 132 to optical signals withtheir respective wavelengths, which are outputted from output terminals194, 195 and 196 by optical receivers 121, 122 and 123, respectively.

In such wavelength multiplex transmission system 171, crosstalk betweenwavelengths can occur at optical transmission line 151 or wavelengthseparation filter 132. When crosstalk occurs, crosstalk components aresuperposed on the signal, and this can lead to deterioration of theoptical signal (e.g. see non-patent literatures 1 to 7).

As an example, FIG. 2 shows flows of signals when three wavelengths arewavelength-multiplexed. Signals 7 a, 7 b and 7 c are electrical signalsinputted into wavelength multiplex transmission system 171. Theseelectrical signals are inputted from input terminals 191 into opticaltransmitter 111, inputted from input terminals 192 into opticaltransmitter 112, and inputted from input terminals 193 into opticaltransmitter 113, respectively. Optical signals 7A, 7B and 7C areoutputted from the respective optical transmitters and transmitted. Intransmission of the optical signals, if there is no crosstalk amongwavelengths of optical signals 7A, 7B and 7C, then signals 7 a, 7 b and7 c are outputted from optical receivers 121, 122 and 123, respectively.

FIG. 3 shows flows of signals in a three-wavelength multiplexedtransmission system when there is crosstalk in transmission of opticalsignals. Signals 7 a, 7 b and 7 c are electrical signals inputted intowavelength multiplex transmission system 171. These electrical signalsare inputted from input terminal 191 into optical transmitter 111,inputted from input terminal 192 into optical transmitter 112, andinputted from input terminal 193 into optical transmitter 113,respectively. Optical signals 7A, 7B and 7C are outputted from therespective optical transmitters and transmitted. When there is crosstalkamong wavelengths of optical signals 7A, 7B and 7C, crosstalkcomponents, in addition to electrical signals 7 a, 7 b and 7 c, areoutputted from optical receivers 121, 122 and 123, respectively. Thatis, from output terminal 194 of optical receiver 121, electrical-levelcrosstalk components 9 ba and 9 ca from optical signals 7B and 7C areoutputted in addition to signal 7 a. From output terminal 195 of opticalreceiver 122, electrical-level crosstalk components 9 ab and 9 cb fromoptical signals 7A and 7C are outputted in addition to signal 7 b. Fromoutput terminals 196 of optical receiver 123, electrical-level crosstalkcomponents 9 ac and 9 bc from optical signals 7A and 7B are outputted inaddition to signal 7 c. Here, reference symbols 9BA, 9CA, 9AB, 9CB, 9ACand 9BC in the figure denote optical crosstalk components.

Crosstalk among wavelengths can be caused by stimulated Raman scattering(SRS), cross phase modulation (XPM) and the like, which are due tononlinearity of optical fiber constituting the optical transmission line(e.g. see non-patent literatures 2, 3, 5 and 7). Crosstalk amongwavelengths can also be caused by poor wavelength separationcharacteristic of the wavelength separation filter, in addition to thenonlinearity of optical fiber (e.g. see non-patent literature 4).

As a method for reducing such crosstalk, crosstalk can be reduced bymaking polarization directions of adjacent optical signals orthogonalwhen multiplexing and transmitting over an optical transmission line(e.g. see patent literature 1). However, this method is effective onlyfor adjacent wavelengths, and no effect of reducing crosstalk has beenobtained for non-adjacent wavelengths.

[Patent Literature 1] Japanese Patent Application Laid-Open No. 08-18536

[Non-Patent Literature 1] K. Kikushima et al., “Signal crosstalk due tofiber nonlinearity in wavelength multiplex SCM-AM TV transmissionsystems,” Optical Fiber Communication Conference (OFC '95),Post-deadline paper, PD24, February to March 1995

[Non-Patent Literature 2] A. Li et al., “Experimental confirmation ofcrosstalk due to stimulated Raman scattering in WDM AM-VSB transmissionsystems,” Electronics Letters, vol. 31, No. 18, pp. 1538-1539, August1995

[Non-Patent Literature 3] Takachio et al., “Review of 10 Gb/s 8CH WDMTransmission System in Arrangement of Wavelengths at IrregularIntervals,” Technical Research Report of the Institute of Electronics,Information and Communication Engineers, CS96-43, pp. 19-24, June 1996

[Non-Patent Literature 4] K-P Ho et al., “Demultiplexer crosstalkrejection requirements for hybrid WDM system with analog and digitalchannels,” IEEE Photonics Technology Letters, Vol. 10, No. 5, pp.737-739, May 1998

[Non-Patent Literature 5] M. R. Phillips et al., “Crosstalk due tooptical fiber nonlinearities in WDM CATV lightwave systems,” IEEEJournal of Lightwave Technology, Vol. 17, No. 10, pp. 1782-1792, October1999

[Non-Patent Literature 6] Shibata et al., “Optical Picture DeliverySystem With FM Batch Conversion Method,” The Journal of the Institute ofElectronics, Information and Communication Engineers, B, Vol. J. 83-B,No. 7, pp. 948-959, July 2000

[Non-Patent Literature 7] F. Coppimger et al., “Nonlinear Ramancrosstalk in a video overlay passive optical networks,” Optical FiberCommunication Conference (OFC 2003) TuR5, March 2003

[Non-Patent Literature 8] ITU-T Recommendation G. 983.3 “A broadbandoptical access system with increased service capability by wavelengthallocation”

DISCLOSURE OF THE INVENTION

The present invention is directed to provide wavelength multiplextransmission systems capable of reducing crosstalk among wavelengths inwavelength multiplex transmission and reducing deterioration of opticalsignals, and transmission apparatuses and receiving apparatuses appliedto such wavelength multiplex transmission systems.

In a wavelength multiplex transmission system according to the presentinvention, transmission apparatus differentially divide one input signalinto two, each of which is converted to an optical signal, andwavelength-multiplexed for transmission. When crosstalk occurs in thiswavelength multiplex transmission system, the crosstalk is superposedonto the two optical signals. This crosstalk will be equally superposedonto each of the signals with inverted polarities. Accordingly, byconverting the optical signals to electrical signals and thendifferentially combining them at a receiving apparatus, their signalcomponents will be accumulated, while their crosstalk components will becancelled out. Thereby, it can realize a wavelength multiplextransmission system capable of reducing deterioration of optical signalsdue to crosstalk, and a transmission apparatus and a receiving apparatusthereof.

Specifically, a wavelength multiplex transmission system according tothe present invention comprises a transmission apparatus and a receivingapparatus connected via an optical transmission line; wherein thetransmission apparatus comprises (N+M) optical transmitters (where N isan integer of 2 or more and M is an integer between 1 and N) fortransmitting input signals as optical signals with differentwavelengths, M differential dividers for differentially dividing Minputsignals out of the input signals, respectively and inputting thedifferentially divided signals into 2×M optical transmitters out of the(N+M) optical transmitters, respectively, and a wavelength multiplexfilter for multiplexing and outputting the (N+M) optical signals fromthe (N+M) optical transmitters, and the receiving apparatus comprises; awavelength separation filter for separating the wavelength-multiplexedoptical signals to output (N+M) optical signals, (N+M) optical receiversfor receiving the (N+M) optical signals from the wavelength separationfilter, respectively, to output signals, and M differential combiners,each differentially combining the output signals from two opticalreceivers receiving a pair of optical signals that has differentiallydivided and transmitted, out of the (N+M) optical receivers, to outputone signal.

For M signals to be differentially divided and transmitted out of Nsignals to be transmitted, the transmission apparatus converts 2×Melectrical signals having their polarities inverted each other tooptical signals together with the other (N−M) signals to transmit. Then,at the receiving apparatus, after the 2×M optical signals are eachconverted to electrical signals, for each pair of signals with invertedpolarities, the signals are differentially combined to provide Msignals. Crosstalk components are equally superposed on the signalsoutputted from the optical receivers, and the difference between thecross components is considered small. Therefore, by converting theoptical signals to electrical signals and then differentially combiningthem at the receiving apparatus, their signal components areaccumulated, while their cross talk components are cancelled out.Accordingly, a wavelength multiplex transmission system can be providedwhich reduces crosstalk occurred between a differential divider and adifferential combiner, reducing deterioration of signals. Note that theoptical receiver here is not the one that identifies temporal andamplitude scales from a received optical signal to re-construct thesignal.

The transmission apparatus is characterized in that two correspondingsignals from one differential divider are inputted into two opticaltransmitters, respectively, and transmitted as optical signals withadjacent wavelengths. Crosstalk arisen from nonlinearity of opticalfiber used for an optical transmission line will have similar influenceas the wavelengths of the two optical signals approach each other.Accordingly, by transmitting a pair of differentially divided signals asoptical signals of adjacent wavelengths, it can make crosstalkcomponents superposed on the signals outputted from optical receiversbecome similar. Thereby, a wavelength multiplex transmission system canbe provided which effectively reduces crosstalk due to nonlinearity ofoptical fiber.

The receiving apparatus is characterized in that it further comprises adelay time controller for adjusting delay time difference between thepair of optical signals over the optical transmission line, at apreceding stage of the differential combiner. The delay time differencestems mainly from wavelength dispersion. The delay time controlleradjusts difference in delay time for the pair of signals with invertedpolarities outputted from the differential divider to arrive at thereceiving apparatus. Thereby, the signals can be accumulated withtemporal alignment of their signal components outputted from the opticalreceivers, so that even if the optical signals are deteriorated bywavelength dispersion, their crosstalk components can be cancelled outwith differential combining and the signal can be effectivelyreproduced. Accordingly, a wavelength multiplex transmission system canbe provided which reduces crosstalk and reduces deterioration of opticalsignals even in high-density wavelength multiplex transmission orbroadband wavelength multiplex transmission where an optical signal isdeteriorated due to wavelength dispersion.

A transmission apparatus according to the present invention comprises;(N+M) optical transmitters (where N is an integer of 2 or more and M isan integer between 1 and N) for transmitting input signals as opticalsignals with different wavelengths, M differential dividers fordifferentially dividing Minput signals out of the input signals,respectively, and inputting the differentially divided signals into 2×Moptical transmitters out of the (N+M) optical transmitters,respectively, and a wavelength multiplex filter for multiplexing the(N+M) optical signals from the (N+M) optical transmitters to output.

For M signals to be differentially divided and transmitted out of Nsignals to be transmitted, the transmission apparatus converts 2×Melectrical signals having their polarities inverted each other tooptical signals together with the other (N−M) signals to transmit. Whencrosstalk occurs over the optical transmission line, after the 2×Msignals are each converted to electrical signals at the receivingapparatus, for each pair of signals having inverted polarity oneanother, the signals are differentially combined to provide M signals.Crosstalk components are equally superposed on the signals outputtedfrom the optical receivers, and the difference between the crosscomponents is considered small. Therefore, by the receiving apparatusconverting the optical signals to electrical signals and differentiallycombining them, their signal components are accumulated, while theircross talk components are cancelled out. Thereby, a transmissionapparatus can be provided which reduces crosstalk occurred between adifferential divider and a differential combiner, reducing deteriorationof signals.

The transmission apparatus is characterized in that two correspondingsignals from one differential divider are inputted into two opticaltransmitters, respectively, and transmitted as optical signals withadjacent wavelengths. Crosstalk due to nonlinearity of optical fiberused for the optical transmission will have similar influence as thewavelengths of the two optical signals approach each other. Accordingly,by transmitting a pair of differentially divided signals as opticalsignals of adjacent wavelengths, it can make crosstalk componentssuperposed on the signals outputted from the optical receivers becomesimilar. Thereby, as for the transmission apparatus, a transmissionapparatus can be provided which effectively reduces crosstalk due tononlinearity of optical fiber.

A receiving apparatus according to the present invention ischaracterized in that it comprises; a wavelength separation filter forseparating a wavelength-multiplexed optical signal to output (N+M)optical signals (where N is an integer of 2 or more and M is an integerbetween 1 and N), (N+M) optical receivers for receiving the (N+M)optical signals from the wavelength separation filter, respectively, tooutput signals, and M differential combiners for differentiallycombining the output signals from two optical receivers receiving pairedoptical signals out of the (N+M) optical receivers, to output onesignal.

At the receiving apparatus, when inputted with 2×M optical signalsconsisting of pairs of optical signals inverted in polarity, after the2×M signals are each converted to electrical signals, for each pair ofsignals with inverted polarities, the signals are differentiallycombined to provide M signals. Crosstalk components are equallysuperposed on the signals outputted from the optical receivers, and thedifference between the cross components is considered small. Therefore,by the receiving apparatus converting the optical signals to electricalsignals and differentially combining them, their signal components areaccumulated, while their crosstalk components are cancelled out.Thereby, crosstalk is reduced which has occurred from the transmissionapparatus to the receiving apparatus, enabling the provision of areceiving apparatus which reduces deterioration of signals. Note thatthe optical receiver here is not the one that identifies temporal andamplitude scales from a received optical signal to re-construct thesignal.

The receiving apparatus is characterized in that it further comprises adelay time controller for adjusting delay time difference between a pairof optical signals over the optical transmission line, at a precedingstage of the differential combiner. The delay time difference stemsmainly from wavelength dispersion. The delay time controller adjustsdifference in delay time for a pair of signals with inverted polaritiesoutputted from the differential divider to arrive at the receivingapparatus. Thereby, the signals can be accumulated with temporalalignment of their signal components outputted from the opticalreceivers, so that even if the optical signals are deteriorated bywavelength dispersion during the wavelength multiplex transmission,their crosstalk components can be cancelled out with differentialcombining and the signal can be effectively reproduced. Accordingly, awavelength multiplex transmission system can be provided which reducescrosstalk and reduces deterioration of optical signals even inhigh-density wavelength multiplex transmission or broadband wavelengthmultiplex transmission where an optical signal is deteriorated due towavelength dispersion.

The present invention enables the provision of a wavelength multiplextransmission system capable of reducing crosstalk and reducingdeterioration of signals by transmitting a pair of differentiallydivided signals at a transmission apparatus and differentially combiningthe pair of signals at a receiving apparatus, and a transmissionapparatus and a receiving apparatus thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing prior-art configuration of awavelength multiplex transmission system;

FIG. 2 is a schematic diagram illustrating flows of signals in aconventional wavelength multiplex transmission system;

FIG. 3 is a schematic diagram illustrating flows of signals andcrosstalk components in a conventional wavelength multiplex transmissionsystem;

FIG. 4 is a block diagram showing configuration of an embodiment of awavelength multiplex transmission system of the present invention;

FIG. 5 illustrates flows of signal 1 b and crosstalk components fromsignals 1 a and 1 c to signal 1 b in an embodiment of wavelengthmultiplex transmission system of the present invention;

FIG. 6 illustrates another embodiment of a receiving apparatus of thepresent invention;

FIG. 7 shows configuration of another embodiment of wavelength multiplextransmission system of the present invention; and

FIG. 8 shows configuration of another embodiment of wavelength multiplextransmission system of the present invention.

DESCRIPTION OF SYMBOLS

-   1 a, 1 a-, 1 b, 1 b-, 1 c, 1 c-, 7 a, 7 b, 7 c electrical signal-   1A, 1A-, 1B, 1B-, 1C, 1C-, 7A, 7B, 7C optical signal-   4 b, 4 c electrical-level crosstalk component-   4B, 4C optical-level crosstalk component-   9 ba, 9 ca, 9 ab, 9 cb, 9 ac, 9 bc electrical crosstalk component-   9BA, 9CA, 9AB, 9CB, 9AC, 9BC optical crosstalk component-   11, 12, 13, 14, 111, 112, 113 optical transmitter-   21, 22, 23, 24, 121, 122, 123 optical receiver-   31, 131 wavelength multiplex filter-   32, 132 wavelength separation filter-   41, 43, 45 differential divider-   42, 44, 46 differential combiner-   49 delay time controller-   51, 151 optical transmission line-   61, 65, 67 transmission apparatus-   62, 64, 66, 68 receiving apparatus-   71, 72, 73, 171 wavelength multiplex transmission system-   91, 92, 93, 191, 192, 193 input terminal-   94, 95, 96, 194, 195, 196 output terminal

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described below with reference to the drawings.

An example of a wavelength multiplex transmission system will bedescribed with the reference to FIG. 4. FIG. 4 illustrates an example ofconfiguration of wavelength multiplex transmission system 71, where thenumber of signals N to be transmitted is 3, and the number of signals Mfor which crosstalk is to be reduced out of the signals to betransmitted is 1.

Wavelength multiplex transmission system 71 has transmission apparatus61 and receiving apparatus 62 which are connected via opticaltransmission line 51. Transmission apparatus 61 is provided with fouroptical transmitters 11, 12, 13 and 14, one differential divider 41 andwavelength multiplex filter 31. Optical transmitters 11, 12, 13 and 14transmit input signals as optical signals with different wavelengths.Differential divider 41 differentially divides one input signal 1 b outof input signals 1 a, 1 b and 1 c into signal 1 b and signal 1 b- withits polarity inverted, and inputs them to two optical transmitters 12and 13, respectively, out of four optical transmitters 11, 12, 13 and14. Wavelength multiplex filter 31 multiplexes the four optical signalsfrom optical transmitters 11, 12, 13 and 14 to output.

Receiving apparatus 62 is provided with wavelength separation filter 32,four optical receivers 21, 22, 23 and 24, and one differential combiner42. Wavelength separation filter 32 separates the wavelength-multiplexedoptical signal and outputs four optical signals. Optical receivers 21,22, 23 and 24 respectively receive the four optical signals fromwavelength separation filter 32, and output them as electrical signals.Differential combiner 42 differentially combines signals 1 b and 1 b-from two optical receivers 22 and 23 receiving the paired opticalsignals which have been differentially divided and transmitted out offour optical receivers 21, 22, 23 and 24, and outputs one signal 1 b.

Input terminals 91, 92 and 93 are input terminals provided fortransmission apparatus 61, and input signals 1 a, 1 b and 1 c to thewavelength multiplex transmission system are inputted to the respectiveinput terminals. Differential divider 41 generates an inverted signal,having polarity of the inputted signal inverted, and outputs twosignals, i.e. the input signal and the inverted signal. The differentialdivider 41 is the one that can generate differential signals, and may bepart of circuits included in the transmission apparatus. Thedifferential divider 41 may be the one that can vary amplitude of theoutput signal.

Optical transmitters 11, 12, 13 and 14 convert input signals to opticalsignals with different wavelengths, respectively. The wavelength to beoutputted from each optical transmitter may be predetermined.Alternatively, the wavelength to be outputted from the opticaltransmitter may be externally set. By making the wavelength variable,the number of multiplexing wavelengths on optical transmission line 51can be increased adding more optical transmitters or transmissionapparatuses. The optical transmitter may be the one that is capable ofpolarization multiplexing, making polarization of the output opticalsignal variable.

Wavelength multiplex filter 31 multiplexes optical signals with multiplewavelengths and outputs as one optical signal. For wavelength multiplexfilter 31, a conventional wavelength multiplex filter using means forbundling lights, such as a waveguide filter, a coupler or a prism, canbe used. A wavelength multiplex filter applicable to polarizationmultiplexing may be used to achieve higher density inwavelength-multiplexed signal.

Optical transmission line 51 is placed between the transmissionapparatus and the receiving apparatus and conveys an optical signal.Optical transmission line 51 is optical propagation means, such asoptical fiber, a connector and a switch, which can conveywavelength-multiplexed signal. The optical propagation means may includeoptical signal compensation means such as a dispersion compensated fiberand an optical amplifier.

Wavelength separation filter 32 separates wavelength-multiplexed signaland outputs according to wavelengths. A conventionally used wavelengthseparation filter, such as a waveguide filter, a coupler and a prism,can be used as wavelength separation filter 32.

Optical receivers 21, 22, 23 and 24 convert optical signals toelectrical signals. The optical receivers may be the one that can adjustamplitude of the output signal to predetermined amplitude, or may be theone that identifies temporal scale to make synchronization with apredetermined signal. However, the optical receiver is not the one thatidentifies temporal and amplitude scales from a received optical signalto re-construct the signal.

The differential combiner 42 combines a pair of signals that aredifferentially divided by the transmission apparatus. It is the one thatcan perform differential combining and may be part of circuits includedin the receiving apparatus.

In FIG. 4, input signals 1 a and 1 c inputted from input terminals 91and 93 are converted to optical signals 1A and 1C by opticaltransmitters 11 and 14, respectively, and outputted to wavelengthmultiplex filter 31. Input signal 1 b inputted from input terminal 92 isconverted by differential divider 41 to electrical signals 1 b and 1 b-having inverted polarities one another, and are each outputted fromdifferential divider 41. Optical transmitter 12 converts electricalsignal 1 b outputted from differential divider 41 to optical signal 1B.Optical transmitter 13 converts electrical signal 1 b- outputted fromdifferential divider 41 to optical signal 1B-. Wavelength multiplexfilter 31 multiplexes optical signals 1A, 1B, 1B- and 1C outputted fromoptical transmitters 11, 12, 13 and 14, and outputs thewavelength-multiplexed signal to optical transmission line 51.

As described above, in transmission apparatus 61, two signals 1 b and 1b- with inverted polarities are generated by differential divider 41,and converted to optical signals 1B and 1B-, respectively, to beoutputted to optical transmission line 51. On the other hand, inreceiving apparatus 62, optical signals 1B and 1B- are separated bywavelength separation filter 32 and converted to electrical signals 1 band 1 b- by optical receivers 22 and 23. On the signals outputted fromoptical receivers 22 and 23, crosstalk components are equally superposedand the difference between the crosstalk components is considered small.Therefore, by differentially combining these signals, their signalcomponents are accumulated, while their crosstalk components arecancelled out. In this way, by adopting a differential divider in atransmission apparatus, a transmission apparatus can be provided whichis capable of reducing crosstalk occurred between the differentialdivider and a differential combiner, reducing deterioration of signals.

Though, in this example, the number of signals to be transmitted N is 3,and the number of signals to be differentially divided and transmitted Mis 1 out of the signals to be transmitted, N can be any integer of 2 ormore, and M can be any integer between 1 and N. If N is 3 and M is 2,then the numbers of differential dividers and optical transmitters areto be increased by the increased number of M. As such, it is possible toprovide differential dividers and differential combiners and reducecrosstalk only for wavelengths for which influence of crosstalk can be aproblem among wavelength-multiplexed transmission signals.

Furthermore, when both N and M are 3, that is, N differential dividersand N differential combiners are placed for N input signals, crosstalkcan be reduced for all the input signals.

Furthermore, a pair of signals with inverted polarities outputted from adifferential divider may be transmitted using two optical signals havingthe same wavelengths but different polarization directions. Furthermore,the transmission apparatus may be further provided not only with thewavelength multiplexing function but also with a time multiplexingfunction. The number of wavelength multiplex filters is not limited toone. By providing multiple wavelength multiplex filters, it is possibleto transmit wavelength-multiplexed signals with multiple places. Theinput terminals of the transmission apparatus are not limited to theones adopted for electrical signals, and input terminals adopted foroptical signals may be provided.

In transmission apparatus 61, two corresponding signals from adifferential divider are inputted to two optical transmitters,respectively, and transmitted as optical signals preferably withadjacent wavelengths. That is, the wavelengths of optical signalsoutputted from optical transmitters 12 and 13 are wavelength-multiplexedwith the adjacent wavelengths. Crosstalk arisen due to nonlinearity ofoptical fiber used for optical transmission line 51 will have similarinfluence as the wavelengths of the two optical signals approach eachother. Accordingly, optical signals 1B and 1B-converted from thedifferentially divided pair of signals 1 b and 1 b- may be transmittedwith adjacent wavelengths, so that crosstalk components superposed onsignals outputted from optical receivers 22 and 23 can be similar. As aresult, the crosstalk components included in signals 1 b and 1 b- canapproach each other, and thereby reducing crosstalk due to nonlinearityof optical fiber effectively.

In FIG. 4, wavelength separation filter 32 separates awavelength-multiplexed signal transmitted over optical transmission line51 into optical signals 1A, 1B, 1B- and 1C according to theirwavelengths. Optical receivers 21, 22, 23 and 24 converts opticalsignals 1A, 1B, 1B- and 1C separated according to the wavelengths toelectrical signals 1 a, 1 b, 1 b- and 1 c to output. Differentialcombiner 42 differentially combines two signals 1 b and 1 b- fromoptical receivers 22 and 23 to output signal 1 b. Output terminals 94,95 and 96 are output terminals of receiving apparatus 62. Output signal1 a from optical receiver 21 is outputted from output terminal 94;output signal 1 b from differential combiner 42 is outputted from outputterminal 95; and output signal 1 c from optical receiver 24 is outputtedfrom output terminal 96.

As described above, receiving apparatus 62 differentially combines twosignals 1 b and 1 b- outputted from optical receivers 22 and 23 bydifferential combiner 42. Crosstalk components are equally superposed onthe signals outputted from optical receivers 22 and 23, and thedifference between the cross components is considered small. Therefore,by converting optical signals 1B and 1B-to electrical signals 1 b and 1b- and then differentially combining them at receiving apparatus 62,their signal components are accumulated, while their crosstalkcomponents are cancelled out. Thereby, a receiving apparatus can beprovide which is capable of reducing crosstalk occurred between adifferential divider and a differential combiner, reducing deteriorationof signals.

Optical receivers 22 and 23 do not have a function that discriminatestemporal and amplitude scales of a signal component to re-reconstructthe signal. Means for detecting amplitude may be placed at thesubsequent stage of optical receivers 22 and 23. By equalizing theamplitudes of signals 1 b and 1 b-, their crosstalk components canbecome more similar each other, and thereby reducing crosstalkeffectively.

Though, in the example of FIG. 4, M is 1 and N is 3, N can be anyinteger of 2 or more, and M can be any integer between 1 and N. If N is3 and M is 2, then the numbers of the optical receivers and thedifferential combiners are to be increased by the increased number of M.As such, it is possible to provide differential dividers anddifferential combiners and reduce crosstalk only for wavelengths forwhich influence of crosstalk can be a problem among the wavelengthmultiplexed transmission signals.

Furthermore, when both N and M are 3, that is, N differential dividersare provided for N signals, transmitting all the N signals to betransmitted as 2N signals with inverted polarities, which can reducecrosstalk for all the input signals.

The number of wavelength separation filters is not limited to one. Byproviding multiple wavelength separation filters, it is possible toreceive optical signals from multiple places. Some of optical signalsseparated by the wavelength separation filters may be outputted from thereceiving apparatus directly as optical signals. Furthermore, thereceiving apparatus may be the one that can receive and separate atime-multiplexed signal.

FIG. 6 shows another embodiment of a receiving apparatus. FIG. 6illustrates configuration of another receiving apparatus. The differencebetween FIG. 4 and FIG. 6 is that delay time controller 49 for adjustingdelay time difference between the paired optical signals over theoptical transmission line is provided at the preceding stage ofdifferential combiner 42 included in receiving apparatus 64. The delaytime difference arisen mainly due to wavelength dispersion. The outputterminals of optical receivers 22 and 23 are respectively connected tothe input terminals of delay time controller 49, and the output terminalof delay time controller 49 is connected to differential combiner 42.Delay time controller 49 adjusts time delay difference between signals 1b and 1 b- outputted from optical receivers 22 and 23, and outputs thesignal to differential combiner 42. Thereby, signals 1 b and 1 b-differentially divided by differential divider 41 can be aligned in timeto combine.

Accordingly, a receiving apparatus can be provided which is capable ofreducing crosstalk due to wavelength dispersion occurred at opticaltransmission line 51, and reducing deterioration of signals.

FIG. 5 schematically shows flows of signals 1 a, 1 b and 1 c andcrosstalk components when wavelength multiplex transmission system 71shown in FIG. 4 is used. FIG. 5 shows crosstalk components in view ofsignal 1 b. Input signal 1 a inputted from input terminals 91 isconverted to optical signal 1A and then transmitted by opticaltransmitter 11, and is then received and converted from optical signal1A to electrical signal 1 a by optical receiver 21, and is outputtedfrom output terminals 94. Input signal 1 c inputted from input terminals93 is converted to optical signal 1C and transmitted by opticaltransmitter 14, and is then received and converted from optical signal1C to electrical signal 1 c by optical receiver 24, and is thenoutputted from output terminals 96. Input signal 1 b inputted from inputterminals 92 is inputted into differential divider 41, and differentialdivider 41 outputs signal 1 b and signal 1 b- of which polarity isinverted from signal 1 b. Signal 1 b outputted from differential divider41 is converted to optical signal 1B and transmitted by opticaltransmitter 12, and then received and converted to electrical signal 1 bby optical receiver 22. Other electrical signal 1 b- outputted fromdifferential divider 41 is converted to optical signal 1B- andtransmitted by optical transmitter 13, and then received and convertedto electrical signal 1 b- by optical receiver 23. Signals 1 b and 1 b-outputted from optical receivers 22 and 23, respectively, aredifferentially combined by differential combiner 42, and output signal 1b is outputted from output terminals 95.

In each optical receiver 21, 22, 23, 24, a signal having crosstalkcomponents in addition to the optical signal superposed is received, asshown in FIG. 5. To optical receiver 22, in addition to signal 1B,optical-level crosstalk component 4B due to optical signal 1A, 1B- and1C is inputted, and crosstalk component 4B is converted to electricalsignal 4 b. Furthermore, to optical receiver 23, in addition to opticalsignal 1B-, optical-level crosstalk component 4C due to optical signal1A, 1B and 1C is inputted, and crosstalk component 4C is converted toelectrical signal 4 c. As a result, signal 1 b and crosstalk component 4b are outputted from optical receiver 22, and signal 1 b- and crosstalkcomponent 4 c are outputted from optical receiver 23. For simplicity,crosstalk components from optical signals 1A, 1B and 1B- to opticalsignal 1C as well as from optical 1B, 1B- and 1C to optical signal 1Aare omitted.

As such, leakage from each signal is a crosstalk. Therefore,optical-level crosstalk component 4B superposed on signal 1B andcrosstalk component 4C superposed on optical signal 1B- have similarcharacteristics. Accordingly, when signal 1 b and crosstalk component 4b, and signal 1 b and crosstalk component 4 c are differentiallycombined by differential combiner 42, then crosstalk components 4 b and4 c are mostly cancelled out, while signals 1 b and the 1 b- areaccumulated. Since the cancellation of the crosstalk components iscarried out at the subsequent stage of optical receivers 22 and 23,crosstalk can be reduced which have occurred at the wavelength multiplexfilter, the optical transmission line and the wavelength separationfilter. Thereby, crosstalk can be reduced among wavelengths in thewavelength multiplex transmission system, and reducing deterioration ofsignals. Though description has been made with N as 3 and M as 1 here, Ncan be any integer of 2 or more, and M can be any integer between 1 andN.

Description will now be made on an example of a wavelength multiplextransmission system when N is 3 and M is 2, with reference to FIG. 7.FIG. 7 illustrates an example of configuration of wavelength multiplextransmission system 72 which differentially divides and transmits inputsignal 1 a in wavelength multiplex transmission system 71 shown in FIG.4. Wavelength multiplex transmission system 72 comprises transmissionapparatus 65 and receiving apparatus 66 connected via opticaltransmission line 51. In transmission apparatus 65, differential divider43 and optical transmitter 15 are placed in addition to the componentsof the transmission apparatus shown in FIG. 4. Furthermore, in receivingapparatus 66, optical receiver 25 and differential combiner 44 areplaced in addition to the components of the receiving apparatus shown inFIG. 4. Wavelength multiplex transmission system 72 is configured toreduce crosstalk of input signal 1 a in addition to that of input signal1 b.

The flow of input signal 1 a in wavelength multiplex transmission system72 will be described. Input signal 1 a inputted from input terminals 91is inputted to differential divider 43. Differential divider 43differentially divides signal 1 a and outputs signal 1 a and signal 1 a-of which polarity is inverted from signal 1 a. Signal 1 a outputted fromdifferential divider 43 is converted to optical signal 1A andtransmitted to wavelength multiplex filter 31, by optical transmitter11. Signal 1 a- outputted from differential divider 43 is converted tooptical signal 1A- and outputted to wavelength multiplex filter 31, byoptical transmitter 15. Wavelength multiplex filter 31 multiplexesoptical signals 1A, 1A-, 1B, 1B- and 1C, and outputs it to opticaltransmission line 51. Then, the wavelength multiplexed signal inputtedinto optical transmission line 51 is received by receiving apparatus 66and separated according to wavelengths by wavelength separation filter32. Optical signal 1A out of the signals is received by optical receiver21 and converted to electrical signal 1 a. Optical signal 1A- separatedby wavelength separation filter 32 is received by optical receiver 25and converted to electrical signal 1 a-. Signals 1 a and 1 a- outputtedfrom optical receivers 21 and 25 are differentially combined bydifferential combiner 44. Signal 1 a differentially combined bydifferential combiner 44 is outputted from output terminals 94 of thereceiving apparatus.

In wavelength multiplex transmission system 72, input signals 1 a and 1b are differentially divided, and signals 1 a- and 1 b- with invertedpolarities are, together with signals 1 a and 1 b, converted to opticalsignals and transmitted. Crosstalk components are equally superposed oneach of signals outputted from the optical receivers, and therefore, thedifference between the crosstalk components is considered small.Therefore, by differentially combining signals 1 a and 1 a-, and signals1 b and 1 b- at receiving apparatus 66, the signal components areaccumulated, while the crosstalk components are cancelled out, for eachof signals 1 a and 1 b. Since the cancellation of the crosstalkcomponents is carried out at the subsequent stage of optical receivers21, 25, 22 and 23, crosstalk of input signals 1 a and 1 b can be reducedwhich have occurred at the wavelength multiplex filter, the opticaltransmission line and the wavelength separation filter. As such, it ispossible to reduce crosstalk among wavelengths in the wavelengthmultiplex transmission system and reduce deterioration of signals onlyfor wavelengths for which influence of crosstalk can be a problem amongthe wavelength multiplex transmission signals.

Description will now be made on an example of a wavelength multiplextransmission system when N is 3 and M is 3, with reference to FIG. 8.FIG. 8 illustrates an example of configuration of wavelength multiplextransmission system 73 which differentially divides and transmits inputsignal 1 c as well as input signals 1 a and 1 b in wavelength multiplextransmission system 72 shown in FIG. 7. Wavelength multiplextransmission system 73 comprises transmission apparatus 67 and receivingapparatus 68 connected via optical transmission line 51. In transmissionapparatus 67, differential divider 45 and optical transmitter 16 areplaced in addition to the components in the transmission apparatus shownin FIG. 7. In receiving apparatus 68, optical transmitter 26 anddifferential combiner 46 are placed in addition to the components of thereceiving apparatus shown in FIG. 7.

The flow of input signal 1 c of wavelength multiplex transmission system73 will be described. Input signal 1 c inputted from input terminals 93is inputted to differential divider 45. Differential divider 45 outputssignal 1 c and signal 1 c- of which polarity is inverted from that ofsignal 1 c. Signal 1 c outputted from differential divider 45 isconverted to optical signal 1C by optical transmitter 14 and outputtedto wavelength multiplex filter 31. Signal 1 c- outputted fromdifferential divider 45 is converted to optical signal 1C- by opticaltransmitter 16 and outputted to wavelength multiplex filter 31.Wavelength multiplex filter 31 multiplexes optical signals 1A, 1A-, 1B,1B-, 1C and 1C-, and outputs it to optical transmission line 51. Thenthe wavelength multiplexed signal transmitted over optical transmissionline 51 is received by receiving apparatus 68, and separated bywavelength separation filter 32 according to wavelengths. Optical signal1C out of the signals is received by optical receiver 24 and convertedto electrical signal 1 c. Optical signal 1C- separated by wavelengthseparation filter 32 is received by optical transmitter 26 and convertedto electrical signal 1 c-. Signals 1 c and 1 c- outputted from opticalreceivers 24 and 26 are differentially combined by differential combiner46. Signal 1 c differentially combined by differential combiner 46 isoutputted from output terminal 96 of the receiving apparatus.

In wavelength multiplex transmission system 73, all input signal aredifferentially divided, converted to optical signals, transmitted, andthen differentially combined by the receiving apparatus. Crosstalkcomponents are equally superposed on the signals outputted from theoptical transmitters, and therefore, the difference between thecrosstalk components is considered small. Therefore, by differentiallycombining signals 1 a and 1 a-, signals 1 b and 1 b-, and signals 1 cand 1 c- at receiving apparatus 68 after converting all the opticalsignals to electrical signals, their signal components are accumulated,while their crosstalk components are cancelled out. Since thedifferential combining is carried out at the subsequent stage of theoptical receiver, crosstalk can be reduced which have occurred at thewavelength multiplex filter, the optical transmission line and thewavelength separation filter. As such, it is possible to reducecrosstalk among wavelengths in the wavelength multiplex transmissionsystem and reduce deterioration of signals. In this way, if N and M areequal, using N differential dividers and N differential combiners for Ninput signals, crosstalk can be reduced for all the input signals.

The wavelength multiplex transmission systems 71, 72 and 73 may beprovided with a polarization multiplex filter and a polarizationseparation filter so that it transmits a pair of signals with invertedpolarities outputted from the differential divider, using two opticalsignals having the same wavelengths but different polarizationdirections. Furthermore, they may be provided not only with thewavelength multiplexing function but also with a time multiplexingfunction. The number of the wavelength multiplex filters and the numberof the wavelength separation filters are not limited to one. Providedwith multiple filters of such, communications can be realized viawavelength-multiplexed signals with multiple places. The input terminalsto the transmission apparatus and the output terminals from thereceiving apparatus may be the one that are used for transmittingoptical signals.

INDUSTRIAL APPLICABILITY

The present invention is applicable not only a one-way wavelengthmultiplex transmission system but also a two-way wavelength multiplextransmission system in which multiple optical signals are wavelengthmultiplexed in one way. In this case, not only far end crosstalk butalso near end crosstalk can be reduced.

1. A wavelength multiplex transmission system having a transmissionapparatus and a receiving apparatus connected via an opticaltransmission line to transmit an input signal, wherein: the transmissionapparatus is configured to convert differential signals of the inputsignal to separate optical signals to transmit to the opticaltransmission line; and the receiving apparatus is configured to receivethe separate optical signals from the optical transmission line toreproduce the differential signals and further configured to adjust atime difference between the reproduced differential signals.
 2. Thewavelength multiplex transmission system according to claim 1, whereinthe receiving apparatus further combines the reproduced differentialsignals to reproduce the input signal.
 3. A wavelength multiplextransmission system having a transmission apparatus and a receivingapparatus connected via an optical transmission line, wherein: thetransmission apparatus comprises: (N+M) optical transmitters (where N isan integer of 2 or more and M is an integer from 1 to N) fortransmitting a plurality of input signals as optical signals withdifferent wavelengths, M differential dividers for differentiallydividing M input signals out of the plurality of input signals,respectively, and inputting the differentially divided signals into 2×Moptical transmitters out of the (N+M) optical transmitters,respectively, and a wavelength multiplex filter for wavelengthmultiplexing and outputting the (N+M) optical signals from the (N+M)optical transmitters, and wherein: the receiving apparatus comprises: awavelength separation filter for separating the wavelength multiplexedoptical signals to output (N+M) optical signals; (N+M) optical receiversfor receiving the (N+M) optical signals from the wavelength separationfilter, respectively, to output (N+M) output signals; M differentialcombiners, each differentially combining the output signals from the twooptical receivers receiving a pair of optical signals which have beendifferentially divided and transmitted, out of the (N+M) opticalreceivers, to output one signal; and a delay time controller foradjusting delay time difference between a pair of optical signals on theoptical transmission line, at the preceding stage of the differentialcombiner.
 4. The wavelength multiplex transmission system according toclaim 3, wherein the transmission apparatus has two correspondingdifferential signals from one differential divider as inputs to twooptical transmitters, respectively, and transmits the two differentialsignals as separate optical signals with adjacent wavelengths.
 5. Areceiving apparatus for receiving optical signals including differentialsignals of a signal, comprising: optical receiving means for receivingthe optical signals; optical conversion means for reproducing thedifferential signals from the optical signals; differential combiningmeans for combining the reproduced differential signals; and timedifference control means for adjusting time difference between thereproduced differential signals.
 6. The receiving apparatus according toclaim 5, wherein: the differential signals are wavelength-multiplexed asseparate optical signals with different wavelengths; and the opticalconversion means reproduces the differential signals from the separateoptical signals.
 7. A receiving apparatus comprising: a wavelengthseparation filter for separating a wavelength multiplexed optical signalto output (N+M) optical signals (where N is an integer of 2 or more andM is an integer from 1 to N); (N+M) optical receivers for receiving the(N+M) optical signals from the wavelength separation filter,respectively, to output (N+M) output signals; M differential combiners,each differentially combining the output signals from two opticalreceivers receiving a pair of optical signals out of the (N+M) opticalreceivers, to output one signal; and a delay time controller foradjusting delay time difference between the pair of two optical signalson the optical transmission line, at the preceding stage of thedifferential combiner.